1. Field of the Invention
The present invention relates to a method and apparatus for fabricating fiber Bragg gratings (FBGs), particularly, a method and apparatus for fabricating FBGs having a phase shift portion or a change in refractive index modulation amplitude (apodization, for instance).
2. Description of the Related Art
A phase mask method is known as a method for fabricating FBGs by forming Bragg diffraction gratings in a core of an optical fiber. In the phase mask method, the interference light of the ultraviolet laser light is exposed to a core of an optical fiber, thereby forming a periodic refractive index modulation structure in the core of the optical fiber in the longitudinal direction of the optical fiber. See non-patent document 1, Toru Mizunami, “Optical Fiber Bragg Gratings”, OYO BUTURI, Vol. 67, No. 9 (1998), pp. 1029–1034.
FBGs having a phase shift portion are used in optical code division multiplexing (OCDM) encoders and decoders. Super structure FBGs (SS-FBGs) used in an OCDM encoder or decoder have multiple phase shift portions formed in positions depending on the code type. One method for forming a phase shift portion in a core of an optical fiber by means of the phase mask method is to reserve an area for forming a phase shift portion in the diffraction gratings of the phase mask. See non-patent document 2, Akihiko Nishiki et al., “Development of Encoder/Decoder for OCDM using a SSFBG: A Verification of Data-rate Enhancement Method”, THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS (IEICE), Technical Report of IEICE. OFT2002-66 (2002-11), pp. 13–18. This method is suitable for mass production of limited types of products and is not suitable for small-lot production of a wide variety of products, because an expensive phase mask must be prepared for each code type.
One method suitable for small-lot production of a wide variety of products uses the phase mask method and builds a phase shift portion or apodization into FBGs by slightly moving or vibrating the optical fiber in the longitudinal direction by a PZT stage utilizing a piezoelectric (PZT) element. See non-patent document 3, W. H. Loh et al., “Complex grating structures with uniform phase masks based on the moving fiber scanning beam technique”, Optics Letters, Vol. 20, No. 20, Oct. 15, 1995.
When an optical fiber 110 is slightly moved or vibrated in the longitudinal direction (X-axis direction) without moving a phase mask 116, as shown in FIG. 12A, the optical fiber 110 is displaced in the transverse direction (Y-axis direction orthogonal to the X-axis direction) due to yawing or the like of an X-axis travel mechanism, as shown in FIG. 12B. Especially, the SS-FBGs, which have multiple phase shift portions, require the optical fiber 110 to be shifted (slightly moved) several times, so that the displacement of the optical fiber 110 in the Y-axis direction is enlarged. In a single-mode optical fiber, for instance, a core 110a of the optical fiber where refractive index modulation structure is formed has a diameter as small as about 8 μm, and the intensity distribution of the ultraviolet laser light 120 is a Gaussian curve having a steep rising edge and a steep falling edge, as shown in FIG. 12B. Therefore, the displacement of the optical fiber 110 in the Y-axis direction causes the amount of the ultraviolet laser light 120 exposed to the core 110a of the optical fiber to vary greatly, leading to a wide range of variation in amplitude of refractive index modulation provided in the core 110a of the optical fiber.
In the example described in the non-patent document 3, the PZT receives quasi-square-wave linear variations in voltage, and the linearity between the PZT input voltage and the travel amount cannot be maintained due to the hysteresis and creep characteristics. Therefore, it is difficult to design the process for forming desired refractive index modulation structure in the core 110a of the optical fiber.